1. Field of the Invention
The present invention relates to a constant voltage boost power supply, and particularly to a constant voltage boost power supply that is suitable to an internal power supply incorporated in a semiconductor integrated device.
2. Description of the Related Art
The constant voltage boost power supply that produces an output voltage higher than a supply voltage given from the outside is essential in various semiconductor memory devices such as a dynamic memory (DRAM) in which a rewrite operation is required, a nonvolatile memory (EEPROM) in which information is stored by accumulating a charge in a floating gate, a fuse memory in which information is stored by fusing a fuse link, and an anti-fuse memory in which information is stored by breaking a gate insulator. For example, a consumption current reducing technique has been proposed in order to reduce a leak current of a MOS transistor constituting a semiconductor memory device during cutoff. That is, a bulk terminal of the MOS transistor is set to a negative voltage, or the bulk terminal is set to a voltage higher than the supply voltage. Therefore, the constant voltage boost power supply becomes a main component in the highly integrated semiconductor logic circuit.
Generally, good power efficiency, elimination of an external component such as a coil, and mounting in a small chip area are required in the constant voltage boost power supply used in the semiconductor memory devices.
Frequently a Dickson charge pump is used as the constant voltage boost power supply. The Dickson charge pump is an electronic circuit, in which an output voltage higher than a supply voltage or a negative voltage is produced while charge and discharge are repeated between plural capacitors (hereinafter referred to as “pumping capacitor”) connected with rectifying element interposed therebetween.
In the Dickson charge pump, the charge and discharge of the pumping capacitor are repeated to perform a boost operation in synchronization with an externally supplied clock signal or a clock signal that is produced by a ring oscillator or a multivibrator. An on-off control system is adopted in order to keep the obtained output voltage constant. In the on-off control system, a resistance voltage dividing circuit divides the output voltage, the obtained monitor voltage and a reference voltage are compared to each other, the charge pump is operated when the monitor voltage is lower than the reference voltage, and the charge pump is stopped when the monitor voltage is higher than the reference voltage.
A boost power supply based on a current measurement clock frequency control system is proposed as another conventional technique (see “Study of High-Performance Charge Pump Power Supply Circuit”, Tomohiro Matsukawa et al., 17th Karuizawa Workshop for Circuit and System, Apr. 26, 2004). In this boost power supply, advantageously a frequency of the clock signal is automatically adjusted such that efficiency is maximized for a load current. A differential amplifier detects and amplifies a voltage drop caused by a current passed through a current-detecting resistance connected in series to an output, the differential amplifier supplies a control voltage, a voltage-controlled oscillator into which the control voltage is fed supplies a clock signal having a higher frequency when the control voltage is high, and the voltage-controlled oscillator supplies the clock signal having the lower frequency when the control voltage is low, thereby realizing the automatic adjustment of the clock signal frequency.
The output voltage in the current measurement clock frequency control system depends on a supply voltage supplied to the charge pump and an electrical characteristic of a MOS transistor constituting the charge pump. The constant voltage boost power supply cannot be configured by the operation, in which a consumption current of the charge pump is increased in the large load current while the consumption current of the charge pump is reduced in the small load current. Generally a constant voltage boost power supply, in which the voltage is kept constant irrespective of the supply voltage or the electrical characteristic of the element, is demanded as the internal boost power supply for the semiconductor memory device. From this standpoint, the charge pump in which the conventional clock frequency control is performed cannot be used as the internal boost power supply.